Field of the Invention
The invention pertains to a turbine guide and a method for regulating a load cycle process of a turbine, particularly a steam turbine, whereby a maximum permissible material stress due to the load cycle process is taken into account.
In the article "Digital Computer Control System for Turbine Start-Up" by N. Honda, Fn. Kavano, J. Matsumura in Hitachi Review, Vol. 27, No. 7, 1978, a computer system as well as a method for carrying out an accelerated start-up of a steam turbine is described. The start-up process is regulated here by thermal stresses as controlled variables, which are precalculated and serve as control variables for increasing a turbine rotation speed and a coupling of the turbine on to a generator for load transmission. The start-up process is divided into many small times stages, whereby for each time stage the temperature division is solved along the turbine shaft by solving a partial differential equation. If the thermal stresses calculated therefrom lie within a permissible framework, then a corresponding signal is transmitted on to a turbine speed regulating unit or a power regulating unit, depending on whether the turbine is in an acceleration phase in which the rotation speed of the shaft is being increased, or whether the turbine is in a power coupling phase in which the turbine is connected on to the generator and brought up to the desired power capacity. The method as well as the corresponding computer system serve the purpose of achieving the shortest possible start-up time, taking into consideration the permissible material stresses for a certain starting frequency.
In the article "Temperature Guide For Power Plant Turbines" by P. Martin et al. in BWK, Vol. 36, No. 12, 1984, a mechanism is described by which the monitoring of the stress of selected turbine parts takes place. With this mechanism a regulation of each starting sequence takes place, so that the material fatigue over the expected operation period of the turbine remains below a critical value. It is however assumed that a turbine during its period of application goes through about 4000 start-up sequences, out of which about 3000 are hot starts, 700 are warm starts and 300 are cold starts. For the regulation, the target capacity as well as the rated power capacity transient are pre-given. Taking into consideration the measured rotation speed, the heat transfers of steam on to the rotor material are determined. From that the temperature distribution on the rotor is determined and from that again a stress value as a superimposition of thermal and mechanical stresses can be determined. From the total stress within the rotor as well as the valve housing, percentages of the degree of fatigue are calculated from the time position stress and expansion cycle stress and then added up to the total fatigue degree, which is recorded daily. The calculated stress values serve the purpose of controlling the set-up process, whereby the rated temperature transients are pre-given as limiting values.
In the article "Turbine Guide Calculators For Thermal Monitoring Of Steam Turbines" by E. Geller and F. Zerrmayr in Siemens-Energietechnik 4, issue 2, 1982, a turbine guide calculator is described, in which the start-up speed and the power variation speed is controlled under consideration of the material fatigue and simultaneously the material fatigue caused is determined. As a measurement for heat stress one takes the difference between an average temperature T.sub.m and the surface temperature T.sub.1, of a component. For adapting the regulation to different start-up and take-off sequences and for power variations of fixed-pressure operated turbines, three different regulating modes are foreseen, which correspond to a fast, a medium and a slow variation. Depending on the mode, a maximum permissible temperature difference (T.sub.m, T.sub.1) is pre-given as a function of the average temperature T.sub.m. The actual temperature difference in each case is determined by the turbine guide calculator and from that the free amount for maximum permissible temperature difference is calculated. Apart from the calculation of the momentary free amount, a preview of the expected course of the free amount is also carried out. From both of these values a guiding value is formed with the help of which the start-up and stress speed can be changed in advance by the rated value guide for the speed and capacity, and thus an adaptation to the dynamic plant behavior can be achieved. Along with the operation for regulating the start-up or starting sequence as well as a power variation sequence, the life-span consumption from expansion cycle fatigue is calculated, so that one can determine in a timely manner and beforehand, when the time point would be reached at which a precise inspection of the turbine would become necessary. The start-up mode "normal" corresponds exactly to a start-up mode by which 4000 load cycles of the turbine are possible in a safe manner. The start-up mode "fast" leads to a higher stress corresponding to about 800 possible load cycles and the start-up mode "slow" leads to a lower material fatigue, so that in this case about 10000 load cycles are possible safely.